Background: The Duckworth-Lewis (D/L) method of revising target scores in Limited Overs Internationals was first used early in 1997.
The virtue of this rule is that, unlike other methods of correction in use, it maintains the balance of the game across a suspension in play.
If one team is in the ascendancy when play is halted and the match has to be shortened, that team is still in the ascendancy, and by just as much, when play can resume with fewer overs remaining.
Concept: The method is based on the concept that a match starts with both teams possessing the same resources to build their innings.
The teams have a fixed number of overs to receive, usually 50 in international matches, and they have 10 wickets to lose.
As the innings progresses these two resources are gradually used up and a single table of figures gives the percentages of the run scoring resources of the innings that remain for all possible combinations of overs left and wickets lost.
The table has been constructed from a detailed study of the scorecards from several hundreds of one-day matches, mainly internationals, played over recent years.
If an innings is interrupted and has to be shortened, the table is used to find out what percentage of the resources is lost.
Any shortening of the match after it has started upsets the balance of resources and a revised target is necessary to compensate the team that has suffered more.
This is the case even if both the two teams end up with the same number of overs to receive.
The target adjustment is based on the relative run scoring resources available to the two sides after the resources lost by each team are taken into account.
The table reflects the fact that the resources lost by a loss of a certain number of overs depend on: How many overs remained, and how many wickets were down at the time.
For instance, a loss of overs near the end of an innings, especially when there are plenty of wickets still in hand, is usually a far greater loss of resource than the same loss of overs at the beginning of an innings.
Methodology: In the most common situation of a target revision, the team batting second (Team 2) will have less resources for its innings than Team1. In this case Team 2 will be set a reduced target based on the smaller amount of resources it had for its innings compared to Team 1.
But if an interruption occurs during Team 1's innings, it often happens that Team 2 ends up with more resources for its innings than Team 1 has had and in this case the target is revised upwards to compensate Team 1 for the way it was more disadvantaged by the timing of the stoppage.
This is best understood by considering the case where Team 1's innings is curtailed after 40 of an intended 50 overs and Team 2 has just time to face the same 40 overs.
Team 1 has been pacing its innings to last 50 overs and provided it had wickets in hand and might have expected to have made 60 or 70 runs from the final 10 overs.
Team 2, on the other hand, knew it had only 40 overs to face from the moment it started its innings and was able to take greater risks to achieve a higher scoring rate right from the start.
On an average a team can only make about 20 to 25 fewer runs from a 40-over innings than it can make from a 50-over innings.